The first step is to understand how badly your system has been compromised. Often there may be a mix of issues.
Running an anti-virus software may provide some clues, as may the message you receive from your internet service provider.
However if your system is compromised there is a possibility that the diagnosis you receive may also be compromised or misleading.
These products and services are frequently available on a free trial basis.
In many cases, however, it may not be possible to successfully remove all malware and return the system to a known-safe and stable configuration. If you feel these products or services have not identified the issue or failed to resolve your system's problem, then you may need more specialist advice. Go to the Professional help page for details. These companies offer fixed fee services and home visits to get you safely back online and protected.
You may need to consider backing up your critical data, reformat your hard-drive and re-install a fresh version of operating system and your software.
If you feel this is beyond your abilities or you would would like to obtain specialist advice go to the Professional Help page.